This invention relates to nuclear medicine and, in particular, to the correction of spatial errors in gamma camera alignment by the use of multiple point sources.
Alignment of the image matrix system (the imaging coordinates) with the mechanical system (the detector coordinates) of a nuclear camera is one of the key factors that affect the SPECT image quality. Misalignment of these coordinates can result in spatial errors in the identified locations of radionuclide events, causing lack of clarity in the reconstructed nuclear image. There are potentially three translation errors and three rotation errors that a gamma camera detector may encounter during an event acquisition relative to its correct position. The translation errors are in tangential, axial and radial directions of camera gantry rotation. The rotation axes of the rotation errors are parallel respectively to the tangential, axial and radial directions of gantry rotation.
Currently a single point source measurement method is used in the clinical environment to align the detector head or heads on the gantry. In this method a small radionuclide sample which approximates a point source is placed at an off-axis position in the camera""s field-of-view, and a circular SPECT acquisition is acquired. The center of the spot formed by the point source on each frame in the projection is estimated as the center of mass (centroid) of the whole frame. The spatial coordinates of the point source in a transverse direction are then estimated by fitting the centers of the spots on the frames to a sine curve. The spatial coordinate of the point source in the axial direction is estimated as the average of the center position in that direction. The translation errors in the tangential and axial directions for each frame position are calculated based on the estimated point source position.
When there are no rotational errors in the system, the single point source method is relatively accurate. However, when rotational errors become significant, an error correction method that can detect rotational errors as well as the translational errors is highly desirable.
In accordance with the principles of the present invention, a method for detecting and correcting nuclear medicine camera alignment problems or center-of-rotation (COR) errors is described. The inventive method employs multiple point sources in the field-of-view in order to estimate alignment errors. In a preferred embodiment three or more point sources are used. Preferably, the point sources are positioned so that no two sources share the same transverse plane; and no three sources share an oblique plane parallel to the rotation axis. In one embodiment a SPECT acquisition is performed with the multiple point sources in the field-of-view and system alignment information is extracted from the projection data. In a preferred embodiment the method extracts the center of the spot that each point source forms on the frame at a certain detector rotation angle using Gaussian surface fitting. The spatial coordinates of the point sources are found by fitting the centers to a sine curve in transverse directions or a constant in the axial direction. The two translational and three rotational alignment errors are found at each detector position using a least squared fit based on analysis of the imaging system geometry. The preferred method not only measures tangential and axial translation errors of a detector, but also measures all three rotational errors from a single SPECT acquisition of multiple point sources.